The present invention relates generally to a self-propelled elevator system and, in particular, to a drive apparatus for a self-propelled elevator car.
It is known in elevator systems to utilize a counterweight to balance out the car weight and a portion of the conveyed load. The use of a counterweight reduces the driving power required to move the car. Such a reduction is of particular significance in self-propelled elevators because, in addition to the car weight, the weight of the entire drive equipment mounted on the car adds to the required driving power.
The German patent specification DE 35 23 187 shows a self-propelled building elevator having a drive which is mounted on the top side of a car. A driven friction wheel and a counter wheel are pressed by spring force against opposite sides of a vertically extending travel track. In order to balance the weight of the car and the drive as well as a portion of the load in the car, the car is connected with a counterweight by means of cables guided in the elevator shaft by way of a roller mounted at the top of the shaft. One end of the cables is fastened to the counterweight and the other end is fastened underneath the car.
The German patent specification 1 251 925 shows a self-propelled elevator car with a friction wheel drive which utilizes the splaying principle. Guide wheels and drive wheels are mounted at the comers of the upper side of the car, wherein the drive wheels are pressed against the travel track by a spring force. A cable has one end fastened in the center of the upper side of the car and extends over a pair of rollers mounted at the top of the shaft. The other end of the cable is connected to a counterweight. If the cable breaks, a car holding device is triggered.
Both of the above described drive systems have as a common feature the active application of contact pressure of the friction wheels on the travel track exerted by an appropriately arranged compression spring. Upon a reduction in the friction wheel diameter due to wear, there is a reduction in the contact pressure force applied by the driven friction wheel against the travel track as a consequence.